1. Field of the Invention
This invention relate's to a portable screw-type jack (screw jack) which is a tool for lifting up a heavy load and which is chiefly used for attaching/detaching a tire in a repairing work for a flat tire of automobiles, etc.
2. Related Art
The conventional screw jack is called as a three-stage type screw jack which comprises, as disclosed in Japanese Utility Model Publication No. 3046162, a jack case main body with a cylinder body part vertically disposed at a central area thereof, a threaded shaft, an auxiliary threaded cylinder and an inner cylinder which are all sequentially inserted in the cylinder body part such that the threaded shaft occupies the center. A large bevel gear is placed on a thrust bearing which is disposed on a bed part of the jack case main body, and a threaded shaft with an outer periphery thread is vertically arranged and fixed to the center of the large bevel gear. The auxiliary threaded cylinder and the inner cylinder are arranged on this large bevel gear. The auxiliary threaded cylinder with an inner periphery thread and outer periphery thread, which is to be engaged with the threaded shaft, is disposed at the outer side of the threaded shaft. The inner cylinder with an inner periphery thread, which is to be engaged with the outer periphery thread of this auxiliary threaded cylinder, is disposed at the outer side of the auxiliary threaded cylinder. This inner cylinder is upwardly and downwardly movably slidingly fitted to the inside of the cylinder body part of the jack case main body through the outer cylinder covering the outer side of this inner cylinder. A rotation preventing mechanism provided with a vertical groove and a projection upwardly and downwardly movably sliding fitted to the vertical groove is disposed between every adjacent two of the inner cylinder, the outer cylinder and the cylinder body part of the jack case main body.
By rotating a small bevel gear engaged with the large bevel gear borne by the jack case main body from the downward contraction state in which the auxiliary threaded cylinder and the inner cylinder are receiving in the cylinder body of the jack case main body, the threaded shaft is rotated through the large bevel gear. In accordance with the rotation of the threaded shaft, the auxiliary threaded cylinder and the inner cylinder are moved upwardly and extended. Moreover, in accordance with the rotation of the threaded shaft and the auxiliary threaded cylinder, the inner cylinder and the rotation preventing outer cylinder are moved upward and extended to lift up the body of an automobile with which a receiving fitting at the upper end of the inner cylinder is abutted.
In the above-mentioned conventional screw jack, an auxiliary threaded cylinder and an inner cylinder are sequentially inserted in the cylinder main body of a jack case main body about a threaded shaft in a triplet state. Moreover, a rotation preventing outer cylinder is disposed at the outer side of the inner cylinder, and a rotation preventing mechanism is disposed each between the inner cylinder and the outer cylinder and between the outer cylinder and the cylinder main body. Accordingly, in addition to the auxiliary threaded cylinder and the inner cylinder which are moved upward and downward for lifting up a heavy load such as an automobile, a rotation preventing outer cylinder and a rotation preventing mechanism are needed for preventing the rotation of the auxiliary threaded cylinder and the inner cylinder at the time of upward and downward movement. Thus, the number of component parts is increased and a complicated structure is inevitably required. Moreover, since the auxiliary threaded cylinder and the inner cylinder are placed on the large bevel gear fixed to a lower end of the threaded shaft and adapted to rotate the threaded shaft, the auxiliary threaded shaft and the inner cylinder are reduced in length which results in difficulty in upward and downward (extension and contraction) stroke.